This invention relates generally to a method and apparatus for predicting final coating thickness of articles fabricated in multi-stage, successive coating processes and for selecting appropriate die sizes for producing the same, and more particularly, to a method and apparatus for predicting final coating thickness of magnet wire fabricated with multi-stage undercoating and overcoating operations and for selecting optimum die sizes for multi-stage production of magnet wire.
Some articles of manufacture, such as for example, magnet wire used in a manufacture of AC induction motor windings, are fabricated in multiple and successive coating stages. For instance, at least one type of magnet wire includes a copper or aluminum wire coated with, for example, four undercoat layers of a first enamel material and two overcoat layers of second enamel material to insulate and protect the wire and provide the wire with desirable properties for use in motor winding fabrication for AC and DC motors. The undercoat and overcoat layers are successively formed in coating stations that each include a die having a throat of a given diameter that controls a thickness of the enamel coating upon a wire passed through the die. The application of multiple coatings through a plurality of dies is well known in the art. One such apparatus and method for manufacturing such magnet wire is described in U.S. Pat. No. 4,393,809.
Typically, due to difficulties in predicting a final wire diameter after a plurality of coating stages, and further to compensate for manufacturing variability, enamel coatings are applied in excess to ensure that at least a minimum enamel coating thickness is achieved. This typically includes the use of oversized dies, and the sizes of the dies used is often empirically determined by trial and error to consistently achieve specified coating thickness minima. Even so, variability in final wire diameters results in an appreciable amount of wire being scrapped for excessive final diameters, i.e., excessive coating thicknesses. For example, in at least one manufacturing plant, enamel builds averaged 135-140% of specification minima for esterimide amideimide coated M-range copper and aluminum wires, and approximately 10,000 lbs of wire per month was scrapped for oversized final diameter. Thus, not only does the excess enamel coating increase manufacturing costs of magnet wire, but also contributes to unusable product that is ultimately scrapped.
Accordingly, it would be desirable to provide an apparatus and method for predicting final wire diameter of magnet wire produced from a given set of dies in multi-stage coating operations, and further to provide a methodology and apparatus for selecting die sizes to more efficiently achieve a uniformly acceptable final wire diameter with reduced amounts of coating and reduced scrap of the final product. Therefore, manufacturing input of enamel coating may be reduced and production costs of magnet wire lowered.
A method for determining optimum enamel die sizes for each of a plurality of dies for fabricating magnet wire includes a series of calculations based upon an empirically determined build factor for each of the enamel coatings used in the fabrication process. The build factor is a measure of the effective diameter increase of a wire passed through a coating die when the diameter of the wire entering the die is known and a selected thickness of wet enamel coating exiting the die is known.
Notably, the build factor for a given enamel coating is primarily dependent upon only the solids content of the enamel coating solution. Significantly, it is largely independent of the chemistry of the coating solution. Thus, using only three variables, namely the initial or bare wire diameter, the die size or, more specifically, the size of a restricted opening through the die that governs the coating thickness of wire passed through the die, and the build factor of the applied enamel coating, a relatively simple relationship between a final wire diameter after curing and the die size is expressed as follows:
Dpf(n)=D0(n)+Kpass(n)(Ddie(n)xe2x88x92D0(n))
where Dpf(n)=final wire diameter after the nth pass;
D0(n)=initial wire diameter at the nth stage;
Kpass(n)=build factor for the nth pass; and
Ddie(n)=die throat diameter of the nth stage.
By manipulating this relationship, die set sizes can be selected and enamel build accurately predicted at each of a plurality of fabrication stages through multiple coating dies. Wire is fabricated in accordance with pre-selected specifications with reduced amounts of enamel coating and with virtually no scrapped wire due to oversized final wire diameters.
More specifically, a target final diameter of the magnet wire is calculated after completion of undercoat and overcoat operations, and a target incremental diameter build at each undercoat and overcoat stage of magnet wire production is computed based upon initial wire diameter entering the respective manufacturing stage. The incremental diameter builds are dependent upon the respective target final or overall undercoat diameter for all undercoating stages, the final overall or target overcoat diameter for all overcoating stages, and the number of dies used in the respective undercoating and overcoating fabrication process.
An enamel die size is then computed at each undercoat and overcoat stage of magnet wire production based upon initial wire diameter entering the respective stage, the incremental diameter build for the respective stage, and the build factor for the enamel applied at each respective stage. Die sizes and enamel builds are computed iteratively until an complete die set is found to consistently produce magnet wire within acceptable specification limits while using reduced amounts of enamel coating material.
Therefore, a systematic approach is provided to reduce production costs of articles manufactured in multi-stage coating processes within specification limits.